This invention relates to fuel delivery and control systems for gas turbine engines and, more particularly, to fuel delivery and control systems having the additional capability of utilizing the engine fuel as a heat sink for the engine lubrication system.
Future generations of gas turbine engines will introduce engine control requirements more complex than heretofore experienced. The introduction of variable pitch and other geared fans on gas turbine engines will compound the already difficult task of engine lubrication and, in particular, of lubricant cooling.
The basic problem in gas turbine engine oil cooling is to cool the hot oil scavenged from the various oil sumps to a temperature low enough such that engine parts can dissipate their heat to a relatively cooler oil to avoid exceeding their individual design temperature limits. It is desirable to use the engine metered (consumed) fuel as a heat sink for the oil so that thermal energy is returned to the engine cycle and to avoid the costly installation and vulnerability penalties of oil-to-air coolers.
Typically, gas turbine engines have difficulty in obtaining a low temperature heat sink at reduced power settings due to the reduced metered flow rates through the engine fuel control. Consider the typical fuel delivery and control system for modern engines: generally it comprises a fuel pump, driven by the gas turbine engine rotor, which pressurizes a flow of fuel for delivery to a fuel control. The fuel control includes a metering valve for scheduling the flow of fuel to a series of fuel nozzles for injection into a combustor as a function of predetermined control parameters. A fuel/oil heat exchanger is generally included downstream of the fuel control to cool the hot engine oil while preheating fuel delivered to the fuel nozzles. This results in a heat exchanger fuel inlet temperature which equals the sum of the engine fuel pump inlet temperature and the temperature rise across the pump. The pump temperature rise typically varies from 10.degree. to 100.degree. F. (for take-off and idle conditions, respectively) and all of this heat must be dissipated into the metered fuel.
Further compounding the problem is that the fuel pump is designed to function essentially as a constant displacement pump over the flight envelope and to produce an excess fuel output for all but brief periods such as during engine start. As the fuel requirements of the engine decrease and less fuel is metered through the fuel control, the back pressure on the fuel pump decreases. A pressure regulating valve of the flow bypass variety maintains a constant pressure differential across the fuel control metering valve. Since the fuel pump flow exceeds the level necessary to sustain the engine, the pressure regulating valve opens, as necessary, to bypass the excess pump discharge flow to a low pressure point, typically the pump inlet. As a portion of the fuel recirculates again through the fuel pump, its temperature is further increased in cumulative fashion, as a result of the input of pump work and throttling through the pressure regulating valve, such that the temperature of the fuel at the heat exchanger inlet can be well in excess of 200.degree. F. prior to any addition of lube oil heat. Since, after cooling, the required oil temperature must be in the range of approximately 180.degree. to 300.degree. F. (depending on operating conditions), the fuel pump temperature rise is a major problem for effective oil cooling.
Furthermore, it is usually found necessary to incorporate at least one fuel filter ahead of the fuel control to filter out contaminants from the intricate control mechanisms. While it becomes desirable, in theory, to reduce the temperature of the fuel entering the heat exchanger as previously discussed, care must be taken to keep the fuel temperature above 32.degree. F. in order to prevent freezing of water particles in the filter with subsequent fuel blockage. Thus, in the absence of anti-icing inhibitors in the fuel, a fuel/oil cooling scheme should include provisions for maintaining the fuel temperature entering the filter at a temperature above 32.degree. F.
Additionally, the introduction of variable pitch and other geared fans to gas turbine engines results in gearboxes which must transmit considerable horsepower, consequently requiring large gears and high gear loads. The gear scoring factor (temperature rise above bulk oil temperature) for such an engine might well be in the order of 120.degree. F. So as not to exceed the gear limiting temperatures, it is necessary to cool the bulk oil to a temperature considerably below that normally acceptable for other gas turbine engines. For example, the reduction gear supply oil temperature may be 180.degree. F. while the main engine oil supply requirement is less stringent at approximately 300.degree. F. If all of the engine oil (reduction gearbox plus main engine) must be cooled to the lower value (i.e., 180.degree. F.), considerable difficulty is encountered since there is insufficient fuel heat sink (i.e., cold fuel) available. The problems is to selectively cool the oil such that the main engine lubrication system can operate at a higher, yet still effective, oil temperature than the reduction gearbox.